The present invention relates to a continuous method for the overaging of a hot dip metal coated steel sheet or strip material to improve the formability of the material, which is hard due to the presence of C and N in the form of supersaturated solid solution in the base steel.
Hot dip metal coated steel sheets and strips, including those coated with aluminum, zinc and zinc-iron alloy, are widely used for various applications in which advantageous properties of the respective materials are enjoyed. Among known processes for producing such coated steel materials, continuous coating processes involving an in-line anneal are said to be most advantageous from the view points of productivity and economy. The most typical one is known as Sendzimir process. As is known, such a continuous coating process is usually applied to the cold rolled steel sheet or strip which contains carbon up to 0.15% by weight and nitrogen up to 0.01% by weight.
In Sendzimir process, before a cold rolled steel strip is dipped in a molten coating metal bath, it is heated in a non-oxidizing or weakly oxidizing atmosphere to remove any organic materials, attached to the surfaces of the steel strip, including the rolling mill oil used in the cold rolled steps, and then subjected to a reduction in a reducing atmosphere in order to make the surfaces clean. Such heating steps for cleaning the surfaces of the steel strip can be utilized for annealing. In other words, the surface cleaning steps of Sendzimir process can be substituted for the final annealing steps of the cold rolled steel strip making process. For this reason Sendzimir process is very economical and reasonable. However, the coated steel products obtained by Sendzimir process is very hard and poor in the formability. This is because the hot coated steel product which has left the molten metal bath is continuously rapidly quenched. As is well known in the art, those contents of C and N, which have been present in the hot coated steel product in the state of being dissolved in the base steel, are not allowed to sufficiently precipitate upon rapid quenching so that they remain in the quenched product in the form of supersaturated solid solution, rendering the product hard and poor in the formability.
In order to precipitate such supersaturating C and N thereby to improve the formability of the coated steel products, overaging methods are effective, as proposed in examined Japanese Patent Publication Nos. 43-12968 and 46-10922, published on June 1, 1968 and on Mar. 19, 1971, respectively. In these methods, the coated steel sheet or strip material is wound up in the form of coil, and then subjected to a low temperature box anneal for a prolonged period of time at a temperature ranging between 204.degree. and 454.degree. C. Such time consuming batchwise methods are not convenient for a large scale continuous production.